Dampers and Methods for Performing Measurements in an Autoinjector

ABSTRACT

Systems and methods for measuring and damping forces within autoinjectors in accordance with embodiments of the invention are disclosed. In one embodiment of the invention, an injection system comprises a syringe disposed within a housing, the syringe having a first end and a second end; a needle disposed at the second end of the syringe; a plunger disposed at the first end of the syringe and configured to move toward the second end of the syringe; a stopper disposed between the plunger and the second end of the syringe; a first damper disposed between the plunger and the stopper, such that the first damper is capable of damping a first event occurring between the plunger and the stopper; and a second damper disposed at the second end of the syringe, such that the second damper is capable of damping a second event occurring at the second end of the syringe.

CROSS-REFERENCE TO RELATED APPLICATION(S)

The current application claims priority to U.S. Provisional PatentApplication Ser. No. 62/361,679 entitled “Dampers for Injection Pens,”filed Jul. 13, 2016, and U.S. Provisional Patent Application Ser. No.62/362,203 entitled “In situ Strain and Pressure Measurements in anInjection Pen,” filed Jul. 14, 2016, the disclosures of which are hereinincorporated by reference in their entirety.

FIELD OF THE INVENTION

This invention generally relates to the damping and measurement offorces in autoinjectors. More particularly, this invention relates tomethods for performing in situ strain and pressure measurements inautoinjectors, and damping pressures and strains in autoinjectors.

BACKGROUND

Autoinjectors are commonly used in the pharmaceutical industry. Thesedevices are used both with drugs to be administered in case of emergency(e.g., epinephrine), and with drugs to be administered on a morefrequent basis (e.g., alprostadil, exenatide, and etanercept).Autoinjectors are generally considered to be compact and easy to use,and these fully-automated devices can greatly simplify theadministration of drugs which cannot be administered orally.

Although the specific design of each autoinjector may differ, many ofthe devices employ spring-actuated mechanisms. By pressing a button, asyringe needle is inserted into the patient and the drug is delivered. Asyringe within the autoinjector may include a plunger fitted within acylindrical tube or barrel, along which the plunger may slide to expelliquid. Many autoinjectors contain glass syringes.

SUMMARY OF THE INVENTION

Dampers to efficiently dampen peak pressures and strains in the syringesof autoinjectors, and methods to measure such pressures and strains, inaccordance with various embodiments of the invention are disclosed.

In one embodiment of the invention, an injection system comprises asyringe disposed within a housing, the syringe having a first end and asecond end, wherein the second end comprises an exit opening; a needledisposed at the second end of the syringe; a plunger disposed at thefirst end of the syringe and configured to move along an interior cavityof the syringe toward the second end of the syringe; a stopper disposedbetween the plunger and the second end of the syringe, within theinterior cavity of the syringe; a first damper disposed between theplunger and the stopper, such that the first damper is capable ofdamping a first event occurring between the plunger and the stopper; anda second damper disposed at the second end of the syringe, such that thesecond damper is capable of damping a second event occurring at thesecond end of the syringe.

In a further embodiment, movement of the plunger is spring-actuated.

In another embodiment, at least one of the first damper or the seconddamper is formed with viscoelastic foam.

In a yet further embodiment, the first damper is disk shaped.

In another embodiment, the second damper is cylinder shaped.

In yet another embodiment, the first damper is capable of limiting astrain in the syringe during the first event, and the second damper iscapable of limiting a strain in the syringe during the second event,such that fracture is prevented in the syringe.

In still another embodiment, the syringe is formed of glass.

In a still further embodiment, the needle receives sufficient force fromactivation of the plunger to penetrate human skin.

In a yet further embodiment, the needle is fixed to the syringe, andfluid within the interior cavity of the syringe is capable of exitingthrough the exit opening and a hollow cavity of the needle.

In yet another embodiment, the needle is capable of moving from aretracted position to an advanced position, and fluid within theinterior cavity of the syringe is capable of exiting through the exitopening and a hollow cavity of the needle while the needle is in theadvanced position.

A method for measuring forces in an injection system, according toanother further embodiment of the invention, comprises creating a clearshell with dimensions of an original autoinjector, the clear shell beingelongated and hollow for housing components of the originalautoinjector, including a syringe and a syringe carrier; installing apressure transducer inside the syringe; filling the syringe with fluid;installing at least one strain gauge on an outer surface of the syringe;and assembling an instrumented autoinjector. Assembling the instrumentedautoinjector may be performed by mounting the syringe with the pressuretransducer, the fluid, and the at least one strain gauge into thesyringe carrier; and mounting the syringe carrier into the clear shell.

In still another further embodiment, the method further comprisesremoving a needle from the syringe. Installing the pressure transducermay include inserting at least one magnet wire into the syringe; andconnecting a first end of the at least one magnet wire to at least oneleadwire of the pressure transducer, wherein a second end of the atleast one magnet wire extends out of the syringe through a needleopening of the syringe.

In a still yet further embodiment, the fluid includes deionized water.

In still yet another embodiment, installing the at least one straingauge includes bonding a portion of at least one leadwire of the atleast one strain gauge to the outer surface of the syringe.

In a still further embodiment again, the method further comprisesforming an opening in a syringe carrier of the original autoinjector;wherein the at least one strain gauge includes at least one leadwire.Mounting the syringe into the syringe carrier may include directing theat least one leadwire of the at least one strain gauge through theopening formed in the syringe carrier. Mounting the syringe carrier intothe clear shell may include directing the at least one leadwire of theat least one strain gauge through an opening of the clear shell.

In still another embodiment again, assembling the instrumentedautoinjector is further performed by mounting a power pack of theoriginal autoinjector to the clear shell.

In a yet further embodiment, the method further comprises connecting theat least one leadwire of the pressure transducer and the at least oneleadwire of the at least one strain gauge to signal conditioners.

In another further embodiment, the method further comprises mounting theinstrumented autoinjector onto a fixture.

In still another embodiment, the method further comprises positioning ahigh-speed camera to capture movement of components within theinstrumented autoinjector upon activation of the instrumentedautoinjector.

In still yet another embodiment, the method further comprises activatingthe instrumented autoinjector; and recording data transmitted from thepressure transducer and the at least one strain gauge.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates certain features of an example of a spring-actuatedautoinjector.

FIG. 2 is a diagram showing selected mechanical events during operationof an autoinjector.

FIG. 3 is a space-time diagram for an example of a first event in anautoinjector with no air gap, as observed in accordance with anembodiment of the invention.

FIG. 4 illustrates example pressure and stress waves produced at asecond event in an autoinjector with no air gap, as observed inaccordance with an embodiment of the invention.

FIG. 5 is a chart illustrating example pressures in liquid above a conearea of a syringe mounted in an autoinjector with no air gap, asmeasured in accordance with an embodiment of the invention.

FIG. 6 is a chart illustrating example strains in a glass syringe in anautoinjector with no air gap, as measured in accordance with anembodiment of the invention.

FIG. 7 is a space-time diagram for an example of a first event in anautoinjector with an air gap, as observed in accordance with anembodiment of the invention.

FIG. 8 illustrates an example of growth and collapse of a cavity in thecone area at the first event, and an example of cavitation outside thecone, in an autoinjector with an air gap, as observed in accordance withan embodiment of the invention.

FIG. 9 is a chart illustrating example pressures in liquid above a conearea in an autoinjector with an air gap, as measured in accordance withan embodiment of the invention.

FIG. 10 is a chart illustrating example strains in a glass syringe in anautoinjector with an air gap, as measured in accordance with anembodiment of the invention.

FIG. 11 is a conceptual diagram showing an example of axial, radial andhoop stress.

FIG. 12 is a conceptual diagram showing an example of strain produced bystress.

FIG. 13 is a flow chart illustrating a method for instrumenting anautoinjector in accordance with an embodiment of the invention.

FIG. 14 shows an example of an autoinjector with a clear shell inaccordance with an embodiment of the invention.

FIG. 15 is a schematic of a syringe carrier with a slit in accordancewith an embodiment of the invention.

FIG. 16 is a schematic for a PCB 138M186 tourmaline underwater blastpressure sensor.

FIG. 17 shows a schematic of a pressure transducer mounted inside asyringe in accordance with an embodiment of the invention.

FIG. 18 shows a schematic of a syringe instrumented with strain gaugesand at least one pressure transducer in accordance with an embodiment ofthe invention.

FIG. 19 shows a schematic of an instrumented syringe mounted into asyringe carrier in accordance with an embodiment of the invention.

FIG. 20 shows a schematic of a spring-actuated autoinjector instrumentedwith strain gauges and at least one pressure transducer in accordancewith an embodiment of the invention.

FIG. 21 shows an instrumented autoinjector mounted on a fixture inaccordance with an embodiment of the invention.

FIG. 22 shows an experimental setup for testing an instrumentedautoinjector in accordance with an embodiment of the invention.

FIG. 23 shows three stations where forces can be measured in anexperiment performed in accordance with an embodiment of the invention.

FIG. 24 shows charts illustrating pressure measurements in an experimentperformed without dampers in accordance with an embodiment of theinvention.

FIG. 25 shows charts illustrating strain measurements in an experimentperformed without dampers in accordance with an embodiment of theinvention.

FIG. 26 is a chart illustrating pressure measurements at one station inan experiment performed without dampers in accordance with an embodimentof the invention.

FIG. 27 is a schematic diagram showing a spring-actuated autoinjectorwith dampers.

FIG. 28 shows charts illustrating strain measurements in an experimentperformed with dampers in accordance with an embodiment of theinvention.

FIG. 29 is a chart illustrating pressure measurements at one station inan experiment performed with dampers in accordance with an embodiment ofthe invention.

DETAILED DESCRIPTION

Turning now to the drawings, systems and methods for in situ measurementof pressures and strains within autoinjectors, and damping such forces,are illustrated. In many embodiments of the invention, mechanical eventsexist among the moving components of an autoinjector following itsactivation, particularly but not limited to when high spring forces areused to, for example, inject viscous drugs. These events may includemechanical impacts, as well as accelerations of components, or otheroccurrences that may contribute to mechanical failure resulting fromfracture of, for example, a glass syringe within the autoinjector.

In several embodiments of the invention, a system and method forperforming in situ force measurements within an autoinjector arepresented, so as to characterize the stresses and strains to which thecomponents are subjected. Many embodiments of the invention provide forin situ measurements of the pressure inside the syringe and the strainson the outer walls of the syringe, and this may be accomplished using aninstrumented autoinjector. The mechanical loads may thus becharacterized, potentially allowing for optimization of the design ofthe autoinjectors, validation of mechanical models, development ofinsights into device failures, quality control during manufacturing, andcertification and/or verification of reliability of devices.

In some embodiments of the invention, the outer shell of an autoinjectoris replaced with a transparent replica created using, for example, 3Dprinting or stereolithography, and high-speed video imaging is used toobtain quantitative data on the motion of the pen components. In certainembodiments of the invention, the syringe is modified by removing theneedle, adding a pressure sensor inside, and adding strain gauges on itsouter wall. In many embodiments of the invention, the electrical wiringfor the sensor and gauges are attached and routed through the device,and a fixture is used to hold the pen in place during testing.

According to a number of embodiments of the invention, dampers are usedto damp the mechanical motion of autoinjector components, so as tomitigate events within the device, such as by lowering mechanicalimpacts and/or reducing abrupt acceleration or deceleration of syringecomponents. The dampers may be formed using one of various materials,including but not limited to low-resilience, viscoelastic polyurethanefoam, various other types of foam, low-density rubber, crushablestructures, a frictional damper, and/or a fluidic damper. In certainembodiments of the invention, the dampers include two pieces of foamlocated in regions of mechanical event occurrences between thecomponents of the autoinjector. According to some embodiments of theinvention, the damping system may efficiently damp the deleterious peakpressures and strains in the syringe of an autoinjector.

The measurement of in situ forces within an autoinjector and damping ofsuch forces may have various scientific, medical, commercial,educational and/or other uses. Although specific examples are discussedabove and throughout the present specification, it can be readilyappreciated that various embodiments of the invention may be implementedin many different fields, and are not limited to those particularexamples discussed herein.

Events and Forces Within Autoinjectors

FIG. 1 illustrates some key features of one example of a spring-actuatedautoinjector 100, having a structure similar to that commonly used inautoinjector devices, including but not limited to the SureClickautoinjector manufactured by the SHL Group of Taoyuan City, Taiwan andused by Amgen of Thousand Oaks, Calif. FIG. 1A shows an exterior view110 of the autoinjector 100, including power pack activation button 112,power pack 114, and shell 116. FIG. 1B shows the interior 120 of theautoinjector 100, including spring 122, plunger 124, syringe 126,stopper 128 and carrier 130. In this example, the spring 122 iscontained within the plunger rod 124. The syringe 126 may be filled withliquid drug solution and mounted inside the carrier 130, which may slideinside the shell 116. This carrier 130 can act as a guide to ensureproper motion of the syringe 126. The syringe needle 132 can exitthrough end opening 134 to be inserted into the patient's body fordelivery of the drug.

In some autoinjectors, the needle 132 may be fixed to the syringe 126,and fluid within the interior cavity of the syringe 126 can exit throughthe end opening 134 and a hollow cavity of the needle 132. In otherautoinjectors, the needle 132 may be capable of moving from a retractedposition to an advanced position, and fluid within the interior cavityof the syringe 126 can exit through the end opening 134 and a hollowcavity of the needle 132 while the needle 132 is in the advancedposition.

Referring to FIG. 2, when a user activates (202) the autoinjector 100 bydepressing the power pack button 112, the internal mechanism of thepower pack 114 releases the spring 122. The plunger 124 is thenaccelerated (204) downward as a result of the spring force and impacts(206) the stopper 128. In some autoinjectors according to otherembodiments of the invention, the plunger may be disposed, prior to itsdownward movement, to be already in contact with the stopper. In suchcases, the spring force may nonetheless result in a significantacceleration of the syringe. We refer herein to such a mechanicaloccurrence of an impact, high acceleration, and/or other mechanicalinteraction upon spring activation as the first event 206, and this iswhen the syringe 126, the carrier 130 and the liquid contained insidethe syringe 126 are set into motion. Immediately after the first event,there is a relative motion between the stopper 128 and the syringe 126which results in the production of a pressure wave below the stopper128, inside the liquid within the syringe 126. In cases where anair-filled gap is introduced between the liquid and the stopper 128, forexample, in the process of filling the syringe with the drug solution,the production of this pressure wave in the fluid may take placesimultaneously with the compression of the air gap. The pressure wavewill initially travel downward before it reflects off the convergingsection of the syringe 126 close to the needle 132. This internalpressure can create stress and strain in the syringe 126.

The syringe 126 then continues to accelerate (208) from the springforce. A second event 210 is then observed when the syringe 126 and itscarrier 130 reach their travel limit, and both the liquid and thesyringe 126 stop moving. This sudden stop of the syringe's motion can betransmitted to the liquid, generating a pressure wave. This pressureoriginates from the bottom of the syringe 126 (i.e., close to theconvergent section) and travels toward the top. This internal pressurecan create stress and strain in the syringe 126. The sudden stop of thesyringe 126 can also generate elastic (axial) waves propagating alongthe syringe 126, away from the location of the contact between thesyringe 126 and the carrier 130, which can create additional stress andstrain in the syringe 126. The stress and strains (hoop and axial) inthe syringe 126 that are associated with the second event 210 can be dueto the superposition of the axial mechanical loading of the glass aswell as the internal pressure in the liquid.

In experimental situations, it may be possible to observe a third eventif the syringe 126 and/or carrier 130 rebounds after the second event.However, the strains resulting from this event are typically much lowerthan the strains associated with the preceding two events. In addition,if the first and second events are properly mitigated through the use ofefficient dampers, a significant third event can be avoided. The thirdevent may also potentially be eliminated in clinical situations, as aresult of resistance to the syringe motion created by human tissue uponthe needle penetrating under the skin.

It may be observed in an autoinjector without an air gap between thestopper and the liquid that the first event produces a compression wavein the liquid. This compression wave can initiate the translationalmotion of the syringe upon reaching its bottom end. The pressure wavecan also create stress and strain in the glass syringe. FIG. 3 shows aspace-time diagram for one example of this first event. The second eventmay be observed to produce a compressive stress wave in the barrel ofthe syringe and a tensile stress wave in the tip of the syringe. Theabrupt deceleration of the liquid can also produce a compression wave inthe liquid. FIG. 4 illustrates pressure and stress waves produced at oneexample of the second event. FIG. 5 illustrates example pressures in theliquid above the cone area, and FIG. 6 illustrates example strains inthe glass syringe, for an autoinjector without an air gap.

In an autoinjector with an air gap between the stopper and the liquid,it may be observed that this air gap significantly changes the wavedynamics at the first event. FIG. 7 illustrates a space-time diagram foran example of a first event in an autoinjector with an air gap, withevents numbered 1-5 in the diagram. Event 0 indicates the occurrence ofa first event at which, in this case, the plunger impacts the stopper.At Event 1, the relative motion between the stopper and the syringe issubstantial. The frictional force accelerates the syringe downward andthis creates tension in the cone area resulting into cavitation. AtEvent 2, the slow, isentropic compression of the air gap pressurizes thesyringe. At Event 3, the tension wave produced in the cone reflects atthe air-water interface and can become a compression wave. At Event 4,the pressure increase in the syringe due to the compression of the airgap can cause the cavity in the cone to stop growing and to collapse. AtEvent 5, the violent collapse of the cavity produces shock waves. FIG. 8shows the growth and collapse of a cavity in the cone area at the firstevent and evidence of cavitation outside the cone, with frames separatedby 30 ps. FIG. 9 illustrates example pressures in the liquid above thecone area, and FIG. 10 illustrates example strains in the glass syringe,for an autoinjector with an air gap.

Thus, the relative timing of the syringe's acceleration and thepressurization of the syringe can be significant. When there is no airgap, pressurization and acceleration of the syringe may occur almostsimultaneously. In such situations, the pressures and strains that occurmay result in mechanical failure of the syringe, in especially but notlimited to the conical section of the syringe where sharp pressure wavescan be amplified. When there is an air gap, the pressurization of thesyringe may be delayed. The latter situation can result in transientcavitation and produce shock waves which, in turn, may further causefailure of the syringe.

Stress, like pressure, is a force per unit area. Stresses can beobserved in various directions with respect to an object. For example,in the diagram of FIG. 11, axial stress σ_(zz) may act along the z-axison a plane perpendicular to the z-axis; radial stress σ_(rr) may actalong the r-axis on a plane perpendicular to the r-axis; and hoop stressσ_(θθ) may act along the θ-axis on a plane perpendicular to the θ-axis.A strain may indicate the response of a system to an applied stress.Engineering strain may be defined as an amount of deformation as aresult of an applied force, divided by the initial length of thematerial. Thus, stresses can produce strains. As an example, in FIG. 12,a stress is applied in the y-direction. This stress produces both adeformation Δy along the y-axis, as well as a deformation Δx along thex-axis due to the Poisson effect, in which materials tend to expandperpendicularly to a direction of compression.

According to certain embodiments of the invention, methods for measuringforces within autoinjectors have indicated that the stress and strainsassociated with the first and second events can be substantial. Thus,there is a potential for causing mechanical failure such as but notlimited to fracture of the syringe that is formed of, for example,glass. This may particularly be an issue when high spring forces areused to inject viscous drugs.

The high stresses and strains experienced in the activation of anautoinjector may be associated with transient forces that are aconsequence of the rapid acceleration and deceleration of the syringe.These transient loads are typically not necessary for the operation ofthe device, but rather a consequence of the lack of damping in themechanical operation of the standard autoinjectors. In many embodimentsof the invention, damping of the mechanical motion is used to eliminatethe deleterious peak strains and pressure while maintaining theinjection function of the device.

While events and forces within autoinjectors used for medicationdelivery are described above with respect to FIGS. 1 to 12, the conceptsmay be applicable to various other systems utilizing spring-activatedmechanisms and/or syringe devices. Methods and systems for in situmeasurement of stresses and strains in autoinjectors in accordance witha number of embodiments of the invention are discussed further below.

In Situ Strain and Pressure Measurements in an Autoinjector

According to certain embodiments of the invention, methods and systemsfor measuring the liquid pressure and strains in an autoinjector uponactuation of the device may provide understanding regarding some failuremodes of autoinjectors and/or confirmation of manufacturing quality. Inmany embodiments of the invention, an autoinjector sample isinstrumented for pressure and strain measurements. A pressure sensor,such as but not limited to a piezoelectric pressure transducer, may beinstalled inside the syringe. It may be connected using magnet wiresrouted through the hole for the needle. The magnet wires may be formedof copper or aluminum wire coated with a thin layer of insulation.Multiple strain gauges may be installed on the outer wall of thesyringe. The strain gauges may be connected using magnet wires routedthrough a slit in the syringe carrier. In some embodiments of theinvention, high-speed digital video cameras are used to visualize theplunger, the stopper and the syringe upon actuation, as well as toobserve any transient cavitation taking place in the syringe.

Referring to FIG. 13, in a number of embodiments of the invention, amethod 1300 for instrumenting an autoinjector for force measurementsincludes creating (1302) a clear shell, so as to allow for imaging ofthe moving components. The imaging may provide the ability to verify thesequence of events and timing within the device. Quantitative imageanalysis can enable measurements of the velocities of various componentsin time, the impact velocity between the various components, and theaccelerations of various components. The clear shell may be fabricatedby one of various methods of manufacture, including but not limited tothree-dimensional (3D) printing such as by 3D Systems of Rock Hill, S.C.An example of an autoinjector 1000 with a clear shell 1010 is shown inFIG. 14. The clear shell 1010 may be formed to be completely clear, orpartially clear so as to expose particular components within theautoinjector.

In many embodiments of the invention, an opening is formed (1304) in thesyringe carrier. The opening can allow leadwires of strain gaugesmounted, for example, on the outer wall of the syringe, to exit and beconnected to a signal conditioner or bridge. The opening can be formedin one of various shapes, including but not limited to that of an oblongslit, rectangle, circle, or any formation allowing the exit of therelevant wires. In addition, multiple openings or other configurationsmay be used to allow for the exit of the gauge wires. An example ofsyringe carrier 1020 with an oblong slit 1022 for the leadwires is shownin FIG. 15.

According to several embodiments of the invention, the needle in thesyringe is removed (1306) to allow pressure sensor wires to run throughthe bottom opening of the syringe, from which the needle exits thesyringe. Various methods may be used to remove the needle. One manner ofremoving the needle entails heating the tip of the syringe using, forexample, a blow torch or another suitable heat source. The needle maythen be pulled out of the syringe using, for example, pliers or anysuitable tool. Alternatively, a new syringe may be fabricated without aneedle.

To measure the liquid pressure during functioning of the autoinjector, apressure transducer is installed (1308) in many embodiments of theinvention. One of various pressure transducers may be used, includingbut not limited to a PCB 138M186 tourmaline underwater blast pressuresensor from PCB Piezotronics of Depew, N.Y., a schematic for which isshown in FIG. 16.

In a number of embodiments of the invention, the leadwires of thepressure transducer are cut to a short length, for example, such thatthere are 1.0 to 1.5 inches of leadwires connected to the pressuretransducer. Two magnet wires may be introduced inside the syringe fromthe bottom opening of the syringe. In certain embodiments of theinvention, the magnet wires may be cut short so as to minimize noisecapture, while maintaining a workable length for purposes of installingthe pressure transducer. The length of the magnet wires may be 6 to 12inches in certain embodiments of the invention. The gauge of the magnetwires may be chosen according to the diameter of the bottom opening,which can differ from one syringe to the other. As an example, Americanwire gauge (AWG) 34 or 38 may potentially be used with a syringe of anautoinjector similar to the device shown in FIG. 1. Any insulatingcoating of the wires may be removed at both ends of each magnet wireover a distance of, for example, approximately one quarter of an inch.This can be done using various tools, including but not limited to arazor blade, a solvent and/or heat. The magnet wires may then beconnected to the pressure transducer using, for example, a solderingiron and rosin-core solder.

In several embodiments of the invention, a thin coating of insulatingmaterial may be applied on the connections. The insulating material caninclude one of various types of coatings, including but not limited toliquid polyurethane coating typically used to protect strain gauges suchas the M-Coat A from Micro-Measurements/VPA of Wendell, N.C., as well asnon-conductive epoxy. This protective coating may protect the pressuregauge from being shorted should the two connections come into contactduring experimentation. Additionally or alternatively, the pressuregauge may be positioned so as to prevent contact between the twoconnections.

The pressure gauge may be positioned within the syringe by, for example,pulling on the magnet wires outside the syringe. In many embodiments ofthe invention, the pressure transducer is positioned to float in syringeliquid, enabling a straightforward interpretation of the liquid pressuremeasurement. FIG. 17 shows a simplified schematic of a pressuretransducer 1030 mounted inside a syringe 1024 according to an embodimentof the invention. The leadwires 1032 of the pressure sensor 1030 areconnected to magnet wires 1034 running through the opening at the bottomof the syringe 1024.

In many embodiments of the invention, the syringe is filled (1310) usingdeionized water. Using normal water poses a higher risk of shorting thepressure transducer, which may impair the measurements. The syringe mayalso be filled with a drug solution. Once the fluid is inside thesyringe, a stopper (or piston) 1036 can be introduced from the top 1025of the syringe 1024, as shown in FIG. 18. In a number of embodiments ofthe invention, the bottom opening 1026 of the syringe 1024 is sealedusing one of various materials including but not limited to 5-minuteepoxy. The sealant may prevent the magnet wires 1034 from moving and fixthe position of the pressure transducer 1030 inside the syringe 1024.The sealant may prevent the stopper 1036 from being pushed further downinto the syringe 1024 when performing the experiment, which could damagethe pressure transducer 1030.

As shown in FIG. 18, strain gauges 1040 may be installed (1312),according to many embodiments of the invention, on the outer surface ofthe syringe 1024. The strain gauges 1040 may be placed at variouslocations along the barrel of the syringe 1024, and the locations may bemodified depending on the measurement requirements of a particularexperiment. In certain embodiments of the invention, the strain gauges1040 may be placed away from areas where the first and second eventsoccur, so as to avoid erroneous measurements resulting from directimpact by the device components to the strain gauges. As an example, astrain gauge may be placed close to the converging section of thesyringe to monitor transient cavitation that could take place in thevicinity of the cone. Alternatively, strain gauges could be placed onother components of the autoinjector which are subjected to largestresses and strains.

In accordance with certain embodiments of the invention, to improve theadherence of the strain gauges to the surface of the syringe 1024, localabrasion of the syringe surface may be performed where the gauges willbe mounted. On a glass syringe, this can be performed using one ofvarious tools, including but not limited to a rotary tool with analuminum oxide grinding stone. Following this, the outer wall of thesyringe may be cleaned, conditioned and neutralized. This can beachieved, for example, using various substances such as but not limitedto the CSM-2 degreaser, the MCM-1-A conditioner and the MN5A-1-Mneutralizer from Micro-Measurements/VPA of Wendell, N.C.

Various types of strain gauges may be used, including but not limited tominiature strain gauges such as the C2A-06-015LW-120 fromMicro-Measurements/VPA of Wendell, N.C. The leadwires 1042 of the straingauges 1040 may be cut so as to end, for example, approximately 2 inchesaway from each strain gauge 1040. Any insulating coating may be removedfrom the tips of the leadwires 1042. This can be done using varioustools such as but not limited to a razor blade or a solvent. Use of heatmay damage the strain gauge.

The strain gauges 1040 may be bonded to the outer surface of thesyringe. On a glass surface, one of various bonding substances may beused, including but not limited to that from an M-Bond 200 kit fromMicro-Measurements/VPA of Wendell, N.C. or any equivalent product. Afterbonding each gauge, pressure may be applied on the gauge for a period,such as at least 2 minutes, to maximize adherence to the surface. Thegauge adherence may then be verified to ensure proper bonding to thesurface. This can be done using a variety of methods, such as but notlimited to by trying to lift the corners of the strain gauge 1040 usingfine tweezers.

In certain embodiments of the invention, a protective coating such asbut not limited to a liquid polyurethane coating M-Coat A fromMicro-Measurements/VPA of Wendell, N.C., may be applied on the gauges1040. The leadwires 1042 of the strain gauges 1040 may also be bonded tothe surface of the syringe. This can be done using a variety of methods,such as but not limited to applying a small piece of adhesive tape onthe leadwires 1042, as closely as possible to the strain gauges 1040.Bonding the leadwires 1042 to the syringe surface may aid in preventingthe gauges 1040 from being removed from the syringe surface, should theleadwires 1042 be pulled when being reconnected to a signalconditioner/bridge. FIG. 18 shows a simplified schematic of theinstrumented syringe with the strain gauges 1040 installed.

The autoinjector can be assembled (1314) according to many embodimentsof the invention. Prior to mounting the syringe 1024 in the carrier1020, the leadwires 1042 of the strain gauges 1040 may be wrapped aroundthe surface of the syringe 1024. The syringe 1024 can then be mountedinto the syringe carrier 1020, and tweezers or any other appropriatetool can be used to pull the leadwires 1042 through the opening 1022.FIG. 19 shows a simplified schematic of the resulting assembly.

The leadwires 1042 of the strain gauges 1040 may then be wrapped aroundthe carrier 1020. The syringe carrier 1020 with the syringe 1024 may bemounted inside the clear shell 1010. The leadwires 1042 of the straingauges 1040 may again be pulled through a slot of the shell 1010.Finally, the power pack 1012 can be installed, with an example of theresulting instrumented autoinjector 2000 shown in FIG. 20.

The instrumented autoinjector 2000, according to many embodiments of theinvention, is version of an actual device being tested with minimalmodifications to its components, so as to achieve results as accurate aspossible. Specifically, the instrumented autoinjector 2000 may be testedto measure the liquid pressure following actuation of the device, aswell as the hoop and axial strains on the outer wall of the syringe.

According to a number of embodiments of the invention, to facilitatecontrolling the pen position during testing, the instrumentedautoinjector 2000 may be mounted into a fixture 2010. The specificdesign of the fixture 2010 used to hold the instrumented autoinjector2000 may depend upon the geometry of the autoinjector being studied. Oneexample of a fixture 2010 is built using a number of T-slot extrusionsas shown in FIG. 21. The autoinjector can be mounted in one of variouspositions, including but not limited to vertically with the either endup, horizontally, or at an angle.

The experimental setup may be finalized according to certain embodimentsof the invention. The pressure transducer 1030 and the strain gauges1040 may be connected to signal conditioners using, for example, asoldering iron and rosin-core solder. An accelerometer can be used totrigger a data acquisition system used to perform the test, so as tocause the capture of data only when the autoinjector is activated, andmay, for example, be mounted on the fixture 2010 holding theinstrumented autoinjector 2000. The accelerometer may have an analogoutput that is provided to an A/D converter in the signal conditioningcircuitry. Additionally, a high-speed camera 2020 may be mounted closeto the autoinjector 2000 along with a bright light source 2030. One ofvarious types of video cameras, including but not limited to high-speeddigital cameras such as a Phantom Digital High-Speed Camera from VisionResearch of Wayne, N.J., may be used for visualization during the tests.The final configuration is depicted in FIG. 22.

The mounted instrumented autoinjector 2000 may be actuated for testingin accordance with several embodiments of the invention. The camera 2020may record the actuation of autoinjector 2000 along with movements ofinternal components. The pressure transducer 1030 and strain gauges 1040may sense pressure and strain respectively, and transmit signals to thesignal conditioners. The signal conditioners may manipulate the data,for example, from analog to digital format.

Once the device is actuated, it may not be recommended to reuse the samesyringe a second time. Since there is often lubricant placed on theinside wall of autoinjector syringes, the first shot actuation of thedevice may cause a portion of the lubricant to be removed. As a result,performing more than one test with the same syringe may yield dissimilarresults from one experiment to the other.

The methods of in situ measurement of forces within autoinjectorsaccording to many embodiments of the invention as described above, mayallow for unprecedented accuracy and precision in the results. Althoughexamples of installed pressure transducers and strain gauges arediscussed above and shown in the figures, it may be readily appreciatedthat one type of sensor could be used without another, and that thenumber of different types of sensors used may vary according to therequirements of a specific application in accordance with variousembodiments of the invention.

While methods and systems for performing in situ strain and pressuremeasurements in an autoinjector are described above with respect toFIGS. 13-22, other systems and methods may be utilized as appropriate tothe requirements of a specific application in accordance with variousembodiments of the invention. Pressures and strains in an undampedautoinjector as measured in accordance with some embodiments of theinvention are discussed further below.

Pressures and Strains in an Undamped Autoinjector

As one example of forces within an autoinjector, the pressure within theliquid contained in the syringe along with the hoop and axial strains onthe outer wall of the syringe have been measured for an autoinjectorsimilar to the device shown in FIG. 1. These in situ measurements on theoriginal device, using instrumentation of the autoinjector according toan embodiment of the invention as discussed in the above section, canhelp develop a physical understanding of various mechanical eventswithin the device, and the fluid-structure interaction between thesyringe and the liquid it contains. The measurements may also provide abaseline from which to assess the effectiveness of dampers in mitigatingthe liquid pressure and the strains resulting from the mechanicalevents.

In this particular experiment, the pressure measurements were performedusing a PCB 138M186 piezoelectric pressure sensor designed forunderwater explosions, and the strain measurements were performed usingC2A-06-015LW-120 strain gauges, in an autoinjector instrumented forforce measurements according to an embodiment of the invention asdescribed in the above section. It can be readily appreciated thatvarious other types of sensors may be employed to perform pressure andstrain measurements in an autoinjector, in accordance with variousembodiments of the invention.

The pressure in the liquid contained inside the syringe was measured atthree different locations as shown in FIG. 23: immediately below thestopper (station 1), half-way between the bottom end of the stopper andthe converging section (station 2), and immediately above the convergingsection (station 3).

The pressure traces at the three stations are shown in FIG. 24, witheach corresponding to a different test and performed using a newinstrumented autoinjector, as labeled. The location of the pressuretransducer for each experiment is indicated above each individual plot.The first pressure peak observed at approximately 0 ms was a result ofthe first event. The maximum pressure due to this event wasapproximately 3 to 6 MPa; the exact value depended on the location ofthe pressure measurement and the impact velocity of the plunger on thestopper. The second pressure peak observed approximately 2.5 ms laterwas due to the second event. The maximum pressure due to this event wasapproximately 5 to 6 MPa, and the exact value depended again on thelocation of the sensor and on the impact velocity.

In a separate test, hoop strains (aligned with the circumference of thesyringe) and axial strains (aligned with the axis of the syringe) weremeasured on the outer wall of the syringe. The hoop strains weresimultaneously measured both immediately below the stopper (station 1)and immediately above the converging section (station 3). The axialstrains were only measured immediately above the converging section(station 3). The strain signals are shown in FIG. 25.

The corresponding pressure in this test, measured halfway between thestopper and the converging section (station 2) is shown in FIG. 26. Thespring constant for this test was approximately 500 N/m. The firstpressure peak observed at approximately 0 ms is due to the first event.The maximum pressure due to this event is close to 2 MPa. The secondpressure peak observed approximately 2.5 ms later is due to the secondevent. The maximum pressure due to this event is approximately 4 MPa,and the exact value depends again on the location of the sensor and onthe impact velocity. A third event is also observed in this experimentbecause the syringe rebounded after the second event.

The strains related to the first event are due to the pressure wavetraveling within the liquid, resulting in maximum hoop strains ofapproximately 125 με and maximum axial strains of approximately 250 με.Strains in the conical section of the syringe could potentially belarger, as the geometry of the cone may lead to focusing (i.e.,amplification) of the pressure waves. The strains associated with thesecond event are much larger than the strains associated with the firstevent. Again, the net strains (hoop and axial) are due to thesuperposition of the strains from the axial mechanical loading of theglass as well as the internal pressure waves in the liquid. The maximumhoop strains due to this second event are approximately 320 με and themaximum axial strains are approximately 175 με

It is possible to estimate the order of magnitude of the mechanicalstresses in the glass from σ˜Eε=65 GPa×320 με=21 MPa. From this it maybe concluded that the stresses in the glass could be sufficiently largefor failure to occur.

While pressures and strains measured in an experiment using an undampedautoinjector are described above with respect to FIGS. 23-26, otherexperiments may be conducted as appropriate to the requirements of aspecific application in accordance with various embodiments of theinvention. Dampers for autoinjectors in accordance with a number ofembodiments of the invention are discussed further below.

Dampers for Autoinjectors

In an autoinjector, the injection and needle motion is typicallycontrolled primarily by the quasi-static spring force, rather than bypeak pressures or peak forces due to transients associated withmechanical events that occur during operation of the device. Thus, thelarge peak pressures and strains that occur, such as those reported inthe previous section for the undamped autoinjector, are thereforeunnecessary for device performance and can be damped without affectingthe injection function of the device. According to many embodiments ofthe invention, proper damping of first and second events may reduce theprobability of mechanical failure of the syringe due to peak stresses inthe glass exceeding a failure threshold. This may be achieved by addingdampers within the device to absorb a substantial fraction of themechanical energy, and therefore reduce significantly the impactvelocities, acceleration, and/or deceleration of the components.

According to a number of embodiments of the invention, the dampers areconfigured so as not to modify the behavior of the device in such a waythat it does not serve its main purpose for injection effectivelyanymore. Thus, in certain embodiments of the invention, the dampers maybe configured to meet these five conditions: 1) they effectivelymitigate the first and second events; 2) there is still enough force forthe needle to penetrate the skin; 3) there is sufficient force to injectthe drug solution liquid through the needle with an adequate flow rate;4) the time needed to fully extrude the liquid should not besignificantly increased; and 5) the penetration length of the needleshould not be significantly decreased.

The first condition may be met by using a material for the damper whichexhibits a hysteretic response, or the state of which is dependent onits history (e.g., memory foam). The second condition may be achieved byusing a material which can be compressed over a short period of time bythe actuation mechanism of the autoinjector, but will take a much longertime to relax and return to its initial configuration when the forcesare removed (i.e., material with low resilience), or materials which donot return to their original shape. The third condition may be met byusing a damping material that does not significantly affect thequasi-static spring force, and/or a low-resilience material. The fourthand fifth conditions may be met by using a material with a relativelylarge compliance such that the dampers can be greatly compressed, andthe travel distance of the syringe and the needle is effectivelyunchanged as compared to the case without dampers.

Examples of materials that may be used include, but are not limited to,low-resilience polyurethane foam, also known as LRPU or viscoelasticpolyurethane foam, viscoelastic urethane polymer, neoprene, variousother types of foam, low-density rubber, and/or crushable structuresincluding but not limited to honeycomb structures designed to absorbenergy. In certain embodiments of the invention, a frictional damper maybe used to create sliding friction between the body of the autoinjectorand the moving components. In other embodiments of the invention, afluidic damper may be employed, using fluid viscosity to damp mechanicalmotion. In addition, any materials that are hysteretic, low-resilience,and/or high loss modulus may be desirable for damping. Further, any oneor a combination of various materials that can effectively damp forcescreated by moving components inside an autoinjector may be used, and arenot limited to any specific materials named as examples herein.

Referring to FIG. 27 showing a damping system 2700 according to anembodiment of the invention, a top damper 2710 is formed of a materialwhich exhibits the properties discussed above, and introduced betweenthe bottom end of plunger 124 and the top end of stopper 128. Top damper2710 may effectively damp the first event, such that the peak magnitudeof the pressure wave will be reduced, thus mitigating the hoop and axialstresses in the syringe 126. If a bottom damper 2720 made of, forexample, the same material is also introduced at the bottom end of thesyringe 126, or between the syringe 126 and the shell 116, the peakstresses in the syringe due to the second event may also be mitigated.The exact locations of the top and bottom dampers may vary with thegeometry and properties of a particular autoinjector, and suchadjustments are contemplated within the scope of this invention. Inaddition, one or more of the dampers 2710 and 2720 may potentially bebuilt into the plunger 124, stopper 128, syringe 126, shell 116, or oneor more other parts of an autoinjector.

In many autoinjection devices, the translational motion of the syringecan be controlled and/or limited using a flange and/or shoulder of thesyringe, and/or a stopper. Sometimes the forces may be applied to asyringe carrier into which the syringe is mounted. A damper could beused between the part(s) of the syringe or carrier where a force isapplied to accelerate or decelerate the syringe, and the componentcontacting the syringe to apply this force. Thus, contact points betweenthe syringe (or its carrier) and the driving mechanism, as well ascontact points between the syringe (or its carrier) and a featurelimiting the motion of the syringe, are good candidates for damping tobe applied.

In many autoinjectors, there is a stopper sealing the syringe. Thegeometry of the stopper can vary from one device to the other. When astopper is used, it is often linked to a driving mechanism responsiblefor moving this stopper in order to perform the injection of the liquiddrug and, in some cases, to initiate the translational motion of thesyringe. The contact point between this driving mechanism and thestopper may be a good candidate for damping to be applied.

The dampers 2710 and 2720 may be formed of one or more of various typesof materials that meet the conditions for both effective damping andfunction of the device, as outline above. One example of such a materialis low-resilience polyurethane foam, also known as LRPU or viscoelasticpolyurethane foam. The specific foam used in the experiments reported inthe section below was Pura-Fit 6800 from Moldex of Culver City, Calif.The Pura-Fit 6800 is an ear plug made out of a high-density viscoelasticpolyurethane foam, with an approximate density of 0.23 g/cm3 or 14pounds per cubic foot. In addition, hydraulic dampers may potentially beused. Any one or combination of other materials and mechanisms may beused and/or coordinated together to damp forces in autoinjectors, inaccordance with various embodiments of the invention. Other mechanicalelements, such as but not limited to screws, can be used to deliver thedrug and transport the needle while also minimizing unwantedacceleration or impact forces.

The dampers 2710 and 2720 may be formed into various shapes asappropriate to meet the conditions described above. They may bedisk-shaped, cylindrical and/or formed to fit snugly in the spaces wherethey are to be positioned. As an example and not by way of limitation, atop damper used in an autoinjector similar to the device shown in FIG. 1may be disk-shaped with a diameter just slightly larger than the innerdiameter of the syringe. The height of the disk may be approximately theinitial distance between the plunger and the stopper. Using anuncompressed foam disk of such dimensions would fill the gap between theplunger and the stopper of an autoinjector similar to that shown in FIG.1, prior to firing of the autoinjector. Similarly, as an example and notby way of limitation, a bottom damper used in an autoinjector similar tothe device shown in FIG. 1 may be cylindrically shaped to be insertedbetween the syringe and the shell. A hole centered on the axis ofsymmetry of the cylinder may be made to accommodate the tip of thesyringe and the needle.

While examples of dampers are provided with respect to autoinjectorsimilar to the device shown in FIG. 1, it may be readily appreciatedthat the damping system 2700 is not limited to any particularautoinjector, and may be implemented according to a number ofembodiments of the invention as described above. Further, the exactmechanisms of damping or the details of the autoinjector may vary withinthe scope of the invention. Modifications may be implemented to vary thedensity, geometry, and/or positioning of the damping foam, so as toimprove results.

While dampers for autoinjectors are described above with respect to FIG.27, other devices may be utilized as appropriate to the requirements ofa specific application in accordance with various embodiments of theinvention. Pressures and strains in a damped autoinjector as measured inaccordance with some embodiments of the invention are discussed furtherbelow.

Pressures and Strains in a Damped Autoinjector

Several experiments were performed to assess the effectiveness of thedamping system 2700 as implemented according to some embodiments of theinvention in an instrumented version of an autoinjector similar to thedevice shown in FIG. 1. Although only one such experiment is discussedherein, repeated tests have demonstrated that the results arereproducible and exceedable, with damping of the peak pressure and thepeak strains by as much as 75% being observed. Using in situ measurementtechniques according to some embodiments of the invention as discussedabove, the dampers were shown to significantly reduce peak pressures andstrains in the syringe. Although the peak pressures and strains weresubstantially reduced, the injection function of the device was notsignificantly affected by the presence of dampers.

During this particular testing with a damped version of an autoinjectorsimilar to the device shown in FIG. 1, the bottom end of theautoinjector was rested on a support member 2722 (shown in FIG. 21) madeof nylon. The support member 2722 comprised an inner hole, the diameterof which was approximately 5 mm. The support member 2722 effectivelyblocked a portion of the opening at the bottom of the device, so as tosupport the bottom damper 2720 from below, prevent excessive downwardextrusion of the bottom damper 2720, and provide sufficient dampingforce.

For a test measuring forces within an actuated autoinjector including adamping system according to some embodiments of the invention describedpreviously, measurements were collected at the three stations shown inFIG. 23. The pressure was measured at station 2, the hoop strains weremeasured at station 1 and 3, and the axial strains were measured atstation 3. The spring constant was approximately 545 N/m. Theexperimental results for this test are shown in FIGS. 28 and 29. Theresults for the previous test without dampers as shown in FIGS. 25 and26, are also included in FIGS. 28 and 29 to highlight the effect of thedampers on the strains and internal pressure. The spring constant forthe test without dampers was 500 N/m (about 10% lower than for the testwith dampers).

As can be seen in the strain signals of FIG. 28, the damperssignificantly mitigate the first and second events. In this experiment,the first event was hardly noticeable, and the maximum strains due tothe second event were reduced. The maximum hoop strains wereapproximately 116 με, or reduced by as much as 64% compared to theundamped case. The maximum axial strains were approximately 78 με, orreduced by as much as 69% compared to the undamped case.

Examining the pressure signal shown in FIG. 29, both the first andsecond events were visible. The magnitude of the pressure, however,remained below 2.0 MPa. As a result, the peak pressure was reduced by asmuch as 50%. Another difference between the damped and undamped cases isin the final pressure measured at t=14 ms. In the damped case, thisfinal pressure is approximately 0.6 MPa. In the undamped case, the finalpressure is approximately 0.85 MPa. As an added benefit of damping, thethird event has been successfully eliminated. This can be observed fromthe pressure and strain signals.

Using high-speed imaging, it was also verified that the presence of thedampers did not significantly increase the time needed to extrude theliquid from the syringe despite the lower residual pressure. In fact, nosignificant difference was observed in the injection time when thesyringe contained water. An increase of no more than 10% in theextrusion time was observed when the syringe contained a viscoussilicone oil (viscosity of 5 cSt). The observed variations in extrusiontime are within the test-to-test variations found in the limited testingcarried so far. Using high-speed imaging, it was also verified that thedampers did not reduce the travel distance (i.e., the penetrationlength) of the needle.

According to certain embodiments of the invention, by damping themechanical events within autoinjectors, autoinjectors may be powered bystiffer power packs that enable the use of more viscous drugs and/orsmaller diameter needles. The user experience may also be improved bythe damping of the events, even for low-viscosity drugs, as the dampersmay reduce considerably the sensation of abrupt motion of the device.Significantly, the modification of damping may drastically reducefailures of glass syringes due to event-generated forces.

Although a damped version of an auto-injector similar to the deviceshown in FIG. 1 was used to assess the validity of the damping methodsuggested, the idea can be extended to other designs of autoinjectors.It can be readily appreciated that the materials, placement and geometryof the dampers should be adequately adapted for each autoinjector tooptimally mitigate the mechanical events. Further, while pressures andstrains measured in an experiment using a damped autoinjector aredescribed above with respect to FIGS. 21, 23, 25-26, and 28-29, otherexperiments may be conducted as appropriate to the requirements of aspecific application in accordance with various embodiments of theinvention.

CONCLUSION

Although the present invention has been described in certain specificaspects, many additional modifications and variations would be apparentto those skilled in the art. It is therefore to be understood that thepresent invention can be practiced otherwise than specifically describedwithout departing from the scope and spirit of the present invention.Thus, embodiments of the present invention should be considered in allrespects as illustrative and not restrictive. Accordingly, the scope ofthe invention should be determined not by the embodiments illustrated,but by the appended claims and their equivalents.

What is claimed is:
 1. An injection system, comprising: a syringedisposed within a housing, the syringe having a first end and a secondend, wherein the second end comprises an exit opening; a needle disposedat the second end of the syringe; a plunger disposed at the first end ofthe syringe and configured to move along an interior cavity of thesyringe toward the second end of the syringe; a stopper disposed betweenthe plunger and the second end of the syringe, within the interiorcavity of the syringe; a first damper disposed between the plunger andthe stopper, such that the first damper is capable of damping a firstevent occurring between the plunger and the stopper; and a second damperdisposed at the second end of the syringe, such that the second damperis capable of damping a second event occurring at the second end of thesyringe.
 2. The injection system of claim 1, wherein movement of theplunger is spring-actuated.
 3. The injection system of claim 1, whereinat least one of the first damper or the second damper is formed withviscoelastic foam.
 4. The injection system of claim 1, wherein the firstdamper is disk shaped.
 5. The injection system of claim 1, wherein thesecond damper is cylinder shaped.
 6. The injection system of claim 1,wherein the first damper is capable of limiting a strain in the syringeduring the first event, and the second damper is capable of limiting astrain in the syringe during the second event, such that fracture isprevented in the syringe.
 7. The injection system of claim 1, whereinthe syringe is formed of glass.
 8. The injection system of claim 1,wherein the needle receives sufficient force from activation of theplunger to penetrate human skin.
 9. The injection system of claim 1,wherein the needle is fixed to the syringe, and fluid within theinterior cavity of the syringe is capable of exiting through the exitopening and a hollow cavity of the needle.
 10. The injection system ofclaim 1, wherein the needle is capable of moving from a retractedposition to an advanced position, and fluid within the interior cavityof the syringe is capable of exiting through the exit opening and ahollow cavity of the needle while the needle is in the advancedposition.
 11. A method for measuring forces in an injection system,comprising: creating a clear shell with dimensions of an originalautoinjector, the clear shell being elongated and hollow for housingcomponents of the original autoinjector, including a syringe and asyringe carrier; installing a pressure transducer inside the syringe;filling the syringe with fluid; installing at least one strain gauge onan outer surface of the syringe; and assembling an instrumentedautoinjector by: mounting the syringe with the pressure transducer, thefluid, and the at least one strain gauge into the syringe carrier; andmounting the syringe carrier into the clear shell.
 12. The method ofclaim 11, further comprising: removing a needle from the syringe;wherein installing the pressure transducer includes: inserting at leastone magnet wire into the syringe; and connecting a first end of the atleast one magnet wire to at least one leadwire of the pressuretransducer, wherein a second end of the at least one magnet wire extendsout of the syringe through a needle opening of the syringe.
 13. Themethod of claim 11, wherein the fluid includes deionized water.
 14. Themethod of claim 11, wherein installing the at least one strain gaugeincludes: bonding a portion of at least one leadwire of the at least onestrain gauge to the outer surface of the syringe.
 15. The method ofclaim 11, further comprising: forming an opening in a syringe carrier ofthe original autoinjector; wherein: the at least one strain gaugeincludes at least one leadwire; mounting the syringe into the syringecarrier includes: directing the at least one leadwire of the at leastone strain gauge through the opening formed in the syringe carrier; andmounting the syringe carrier into the clear shell includes: directingthe at least one leadwire of the at least one strain gauge through anopening of the clear shell.
 16. The method of claim 11, whereinassembling the instrumented autoinjector is further performed by:mounting a power pack of the original autoinjector to the clear shell.17. The method of claim 11, further comprising: connecting the at leastone leadwire of the pressure transducer and the at least one leadwire ofthe at least one strain gauge to signal conditioners.
 18. The method ofclaim 11, further comprising: mounting the instrumented autoinjectoronto a fixture.
 19. The method of claim 11, further comprising:positioning a high-speed camera to capture movement of components withinthe instrumented autoinjector upon activation of the instrumentedautoinjector.
 20. The method of claim 11, further comprising: activatingthe instrumented autoinjector; and recording data transmitted from thepressure transducer and the at least one strain gauge.